How to make a force field concentrate only on a specified area?
You can set the Effector Group to limit the force fields which can affect a given particle system:


Add an empty (or really anything that's not a force field)
With the empty selected, create a group (&#9096; CtrlG)
Specify this group as the Effector group for the systems which you do not want to be affected by any force fields:


Optionally delete the empty.


Result: